1. Field of the Invention
This invention generally relates to apparatus for producing an electric potential within a biological environment and more specifically to apparatus for the in situ generation of an electric current in that environment.
2. Description of Related Art
It is known that the application of an electric potential to selected portions in the human body or other biological environments can produce beneficial results. In some procedures, the electric potential produces a current through the selected portions of the body; in others it does not.
For example, the placement of a bimetal structure in the bloodstream constitutes an example of the use of a structure that produces an electrical potential without any current. Electrons migrate to one of the two metals that becomes a cathode and away from the other metal that becomes an anode. The resulting electrical field at the electrodes produces the beneficial result, namely the production of a positive surface charge on the anode that promotes blood clotting.
In accordance with other procedures, an external power source produces a current between spaced electrodes positioned in a biological environment. In one procedure, a platinum electrode, as an anode, is implanted percutaneously in a tumor and another platinum electrode, as a cathode, is implanted percutaneously in tissue at least one tumor diameter from the tumor. Current between the electrodes, generated when the external source is energized, has been shown to promote tumor regression. See B. Nordenstrom, Biologically Closed Electric Circuits (Nordic Medical Publications 1983).
In these and similar procedures, it is necessary to implant each electrode percutaneously, usually using radiographic procedures to assure accurate electrode positioning. Treatment sessions may last for extended times of an hour or more and may be painful to a patient. At the end of the session a surgeon removes the electrodes. If another treatment is necessary, the entire procedure is repeated.
There is also a class of tubular endoprostheses known as "stents" that are well known and have a variety of forms. A stent usually comprises a tubular, radially expanding structure that can be implanted in a vessel to engage and support secondary tissue and maintain vessel patency. Stents may be utilized in body canals, blood vessels, ducts and other body passages and cavities and the term "vessel" is meant to include all such passages and cavities. A stent delivery system typically includes a catheter that supports the stent in a compacted, or low profile, form for transport to a site of implantation. Any of a variety of mechanisms expand the stent radially into the surrounding tissue. After the catheter is removed, the stent retains its expanded shape.
United States Letters Patent No. 4,922,905 of Ernst P. Strecker for a "Dilation Catheter" describes the manufacture, construction and use of certain embodiments of such stents. Strecker's disclosed stent comprises a tubular structure that is knitted from metal or plastic filaments to form a tubular endoprothesis having a wall of loosely interlocked loops. When a physician uses a stent delivery system to properly position the stent, an auxiliary expansion device expands the stent radially causing a plastic deformation of the filament material so the stent retains its expanded shape. My co-pending U.S. application Ser. No. 07/773,847 filed Oct. 9, 1991 for an "Impregnated Stent" discloses a self-expanding stent that does not require an auxiliary expansion device. In these and other stents the filament forms an open mesh wall so the stent has the fluid transport characteristics of a permeable membrane.
Open mesh stents positioned in a vessel proximate a tumor are subject to tumor incursion with consequential partial or full vessel and stent occlusion. If the mesh openings through the stent wall are reduced to 30 microns or less, the stent can prevent cell penetration and prevent occlusion. However, such a stent also has the fluid transport characteristics of an impermeable membrane, so it blocks the transfer of fluid from surrounding tissue into the vessel through the stent wall. In certain vessels, such as the bile duct and urinary tract, such stents can reduce flow rate of a fluid, such as bile or urine, into the vessel from surrounding tissue. These conditions promote fluid crystallization. As a result crystals can form in the vessel and stent and partially or ultimately fully occlude the vessel and stent. Thus, the selection of a conventional stent structure for implantation proximate a tumor is a compromise that must be made in the face of the antithetical problems of tumor incursion and crystal formation.
Notwithstanding the selection of a stent, occlusion eventually occurs either by tumor incursion or crystal formation. The conventional remedial action is to replace the stent or remove the occlusion. As will be apparent, any such remedial action requires traumatic surgery. In many situations patients will not be able to tolerate such remedial actions, so such procedures can not even be considered. Consequently the occlusion must remain.
It has been proposed to resolve the antithetical problems of tumor incursion and crystal formation by using open mesh stent in a variant of the Nordenstrom apparatus. The use of an open mesh stent solves the crystal problem. According to this proposal, the Nordenstrom apparatus would incorporate a metallic stent as one of the two electrodes. Externally applied power would generate a current between the electrodes to cause tumor regression and restore patency through the stent thus overcoming the tumor incursion or slowing the rate of tumor incursion. If this variant is used in a straightforward manner, the first step involves monitoring procedures for determining patency. As a next step, a surgeon implants the stent, second electrode and attendant conductors. The electrodes are energized. After treatment, the surgeon removes at least the second electrode and conductors.
It has been suggested that the stent and second electrode be implanted permanently with the conductors being led to a location where they can be accessed without major surgery. This would minimize patient trauma and facilitate repeated procedures. However, in many applications the difficulty in routing the conductors from the stent to a convenient connection site and the problems of leaving the second electrode proximate the tumor are not readily resolved.